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Diesels Are Even Greener

Written by Bill Siuru

Diesel engines are constantly becoming cleaner. This is great news for the environment, but maybe not for fleet operators. The cleaner 2010 diesel vehicles are more expensive to purchase. They will also require some changes in maintenance procedures and driver training, albeit relatively minor.

Cleaner diesel technology is driven by EPA regulations. The EPA’s Tier 2 Bin 5 requirements for particulate matter (PM)— 0.01 grams per brake horsepower-hour (g/bhp-hr)—are already fully implemented with the 2007 model year engines. This PM reduction required diesel particulate filters (DPF) and ultra-low sulfur diesel (ULSD) fuel, which is now sold everywhere in the U.S.

ULSD, with a sulfur content of less than 15 parts per million (ppm), is required to prevent failure of exhaust after-treatment devices used to meet the EPA 2007 emissions standards. Using diesel with higher sulfur content than ULSD can cause filter clogging, resulting in high exhaust pressures and subsequent expensive engine damage. Because not all the PM is completely oxidized, it can leave inert ash in the filter. DPFs have to be cleaned by blowing or vacuuming out the ash. Manufacturers typically recommend filter cleaning about every 100,000 miles.

Also, only diesel fuel additives clearly labeled as approved for use in model year 2007, or newer engines, should be used.

Cleaner for 2010

For 2010, the rest of the Tier 2 Bin 5 requirements kicked in. Here, the EPA requires NOx (oxides of nitrogen) emissions be reduced to 0.2 g/bhp-hr and NMHC (non-methane hydrocarbons) to be only 0.14 g/bhp-hr. Three methods are being used for NOx emissions, the most popular of which, selective catalytic reduction (SCR), requires another fluid, diesel emissions fluid (DEF), to be carried aboard diesel-powered vehicles. A DOC converts and oxidizes the hydrocarbons into water and carbon dioxide.

DEF, an aqueous solution of about 67.5 percent de-ionized water and 32.5 percent urea, is injected into the exhaust stream upstream of the SCR catalyst where it splits into carbon dioxide and ammonia. The ammonia then converts the NOx into benign nitrogen and water vapor.

When the urea solution is injected into the hot exhaust gas, some ammonia is created that exists for a fraction of a second before it is decomposed again into harmless nitrogen and water in the SCR catalyst. If a trace amount happens to escape, it is captured by the last element in oxidation catalysts specifically designed to decompose it completely.

Drivers will have to be aware of what might happen if they run out of DEF. EPA rules require that the engine must eventually shut down to prevent driving without DEF and thus emitting more NOx. However, this won’t happen suddenly; drivers will get plenty of warning.

For example, Dodge trucks fitted with SCR systems have a low-urea/fill-urea warning that comes on when there are 1,000 miles left. This is followed by more frequent low-DEF warnings, chime sounds and a countdown display of how many miles are left until the tank is empty. If the driver ignores these and runs dry, the next time he stops, the engine will not restart. A warning system with Ford’s new Power-Stroke V8 notifies of the DEF level. Unheeded, speed is eventually limited to 50 mph for a “limp home” mode. If you run out of DEF, the engine will only idle.

DEF will be available at dealers, auto parts stores, truck stops and some gasoline stations. It is already available at Flying J and TA truck stops. It comes in 1- and 2.5- gallon jugs as well as 55-gallon drums for fleets that want to buy it in bulk. Incidentally, onboard SCR systems can determine the composition of the DEF so it cannot be diluted by adding more water. They can also detect homemade mixtures of agricultural- grade urea and demineralized water.

Urea is not harmful to the environment and poses no real hazard if used properly. The aqueous urea solution can be cleaned up with ordinary water if spilled. Urea is widely used as a fertilizer as well as in food products, chewing gum, skin cream and pharmaceuticals.

At temperatures above 120 F, urea will decompose into ammonia, so it should be stored inside cool buildings. If left sitting in the hot sunshine inside vehicles for long periods, it will probably have to be replaced. DEF will freeze at below 12 F, though freezing takes place slowly. DEF can be thawed out, usually by engine heat, without harm. Defrosting systems that melt the DEF shortly after engine startup to enable injection may be necessary in very cold climates to meet the EPA regulations on start times.

Using DEF should not add to the fuel bill. The DEF concentration is typically from 1 to 3 percent of the diesel fuel consumed, depending on the engine and duty cycle. DEF costs between $2.75 to $3.00 a gallon, which is less than diesel fuel.

Better Fuel Economy

Manufacturers are saying that SCRequipped trucks will get better fuel economy because engines can be tuned and calibrated to improve fuel efficiency. Because the catalytic reaction happens after combustion, diesel fuel is also not needed to burn NOx.

Mercedes-Benz, BMW, Audi and Volkswagen are now selling diesel cars that meet the 2010 standards so they can be sold in all 50 states. The approach most are taking is to carry enough DEF on board so refilling can be done at normal service intervals, typically 7,500 miles. On the truck front, Ford, GM, Hino, Isuzu, Volvo, Mack, Detroit Diesel, Freightliner, Caterpillar and Paccar will be using SCR technology.

Which 2010 models, or 2011 in the case of Ford, will have these new, low emission diesel engines requiring DEF? For General Motors, it’s the Chevrolet Silverado 2500HD and 3500HD, Chevrolet Express Cargo van, GMC Sierra 2500HD and 3500HD and GMC Savana Cargo van. For Ford, it’s the F250 Super Duty, F350 Super Duty and E-Series van. The Dodge Ram 2500 and 3500 do not require DEF.

Only a couple of manufacturers are not using SCR. Navistar alone is using advanced exhaust gas recirculation (EGR) in its MaxxForce engines to meet the 2010 NOx requirements, so DEF is not needed. This “cooled exhaust gas recirculation” technology is essentially an extension of technologies used by most engine manufacturers to meet the 2007 requirements.

The third method is to use EGR plus a special “adsorber” catalyst to absorb and break down remaining NOx before it leaves the tailpipe. This method, already used in Cummins 6.7-liter six-cylinder engines for medium-duty Dodges, will be continued in 2010 and later Cummins diesels.

Regardless of the technology, 2010 diesel trucks will be more expensive. For example, Volvo will charge an additional $9,600 for its SCR technology. Hino will add a surcharge of $6,700 per vehicle in 2010 for the additional emission technology.

Most manufacturers say the added maintenance beyond DEF replenishment is minimal. Most SCR systems have a DEF filter. Cummins says the maintenance interval for its filter is about 200,000 miles. Isuzu’s SCR filter has a replacement cycle of one to two years. The only added maintenance for Hinos is the replacement of a DEF sediment filter every 220,000 miles. The unpressurized DEF tank can be made of tough, light plastic to save weight. Plastic tanks and piping preclude rusting.

There is a trade-off between range on a tank of DEF and the added tank weight. This is probably more important to overthe- road cargo trucks than most Class 7 and smaller trucks that come “home” nightly and can be refilled daily if needed. Ford’s new PowerStroke V8 carries sufficient fluid for about 7,500 miles, necessary for those who tow boats and other trailers for long distances.